During hydrocarbon exploration and production, the pore space or “porosity” of a formation is an important property for evaluating the hydrocarbon bearing potential of the formation. Neutron logging measurements are commonly made to obtain a formation porosity estimate. In conventional neutron logging operations, a neutron source emits high energy (“fast”) neutrons into the formation. These fast neutrons are slowed by the surrounding formation (particularly via collisions with hydrogen nuclei present in the formation and the borehole) and eventually captured. The capture of a neutron may result in the emission of one or more prompt capture gamma rays. While, neutron logging tools can be configured to detect the capture gamma rays, epithermal and/or thermal (slowed) neutrons are most commonly detected using neutron detectors deployed on the logging tool.
U.S. Pat. No. 3,483,376 to Locke discloses a system for making neutron porosity measurements. The system includes a neutron source deployed in a tool body in close proximity to and longitudinally spaced from first and second longitudinally spaced neutron detectors (commonly referred to in the art as near and far detectors). The ratio of the neutron count rates measured at the corresponding near and far detectors (the near to far ratio) was found to be more sensitive to formation porosity than to other borehole parameters (e.g., to borehole diameter, borehole shape, or sensor standoff). This ratio has therefore become a common measurement parameter used to compute formation porosity. To this day neutron logging tools commonly make use of axially spaced near and far detectors and the aforementioned near to far ratio to compute neutron porosity.
In general the near to far ratio tends to increase monotonically with increasing porosity. This relationship is commonly (and generally) understood in the industry as follows. Formations having high porosity generally slow down fast neutrons more efficiently than low porosity formations due to the higher concentration of hydrogen in the formation (in the form of water or hydrocarbon in the pore space). In a highly porous formation, the neutrons therefore tend to be captured nearer to the source which typically results in a relatively small number of neutrons being detected at the far detector and therefore a correspondingly high near to far ratio. In less porous formations the emitted neutrons tend to travel farther into the formation resulting in a comparatively higher count rate at both detectors and a correspondingly lower near to far ratio.
While the use of dual (near and far) detectors was intended to minimize the effects of the borehole upon the measured formation porosity, it is well known that neutron porosity measurements continue to be adversely affected by changes in the measurement conditions. For example, borehole size and shape, sensor standoff, drilling fluid weight and salinity, and borehole temperature and pressure are all known to impact the near to far ratio and therefore the neutron porosity measurement. Commercial neutron porosity tools are commonly calibrated for well defined, standard borehole conditions. Variations from these standard conditions can adversely affect the quality of the obtained porosity measurements. Corrections for borehole size and sensor standoff are routinely made to neutron porosity measurements using direct standoff and caliper measurements or standoff and caliper estimates made using various other measurements.
The prior art includes several attempts to improve neutron porosity compensation (or correction) using corresponding ultrasonic standoff and/or caliper measurements. For example, U.S. Pat. No. 4,423,323 to Ellis et al discloses a methodology in which a borehole correction is applied to wireline neutron data. The borehole correction is applied to the neutron data prior to computing neutron porosity and requires a corresponding borehole caliper measurement. U.S. Pat. No. 5,486,695 to Schultz et al discloses a methodology by which LWD sensor data is compensated by applying a standoff weighting factor based on corresponding standoff measurements.
U.S. Pat. No. 5,767,510 to Evans claims to disclose a method for obtaining a neutron porosity measurement that requires no independent measure of borehole geometry. Such “borehole invariance” (as it is termed) is obtained by compensating the far detector so that its borehole sensitivity (referred to as radial sensitivity) matches the borehole sensitivity of the near detector. One drawback with the disclosed method is that such compensation also tends to reduce the sensitivity of the far detector (and therefore the far to near count ratio) to formation porosity. Reduced sensitivity can in turn lead to an unreliable (or noisy) porosity measurement (due to poor statistics). Furthermore, the borehole invariance method requires a knowledge of drilling fluid weight and salinity in order to modify the far detector count rate. As is well known to those of ordinary skill in the art, these drilling fluid parameters are often not well known in-situ.
U.S. Pat. No. 6,894,274 to Valant-Spaight discloses a method in which neutron count rates obtained in water are subtracted from the count rates obtained in the borehole. While this “water compensation” methodology tends to provide improved compensation in low porosity formations, the errors obtained in high porosity formations can be unacceptably large.
Despite the fact that neutron logging techniques have been in commercial use for over 50 years, the interpretation of neutron logs remains challenging and problematic. There is clearly a need in the art for improved methods for making and interpreting neutron logging measurements. In particular there is a need for a method that provides compensation for changes in borehole geometry without requiring measurements thereof (e.g., without requiring corresponding standoff and/or caliper measurements).